(213a) Opportunities and Challenges for Hydrogen Production Using Membranes

The generation of clean electric power and production of low-carbon fuels are urgent needs that must be addressed as the global concentration of anthropogenic carbon dioxide (CO₂) emissions is rapidly increasing. As shown in Figure 1, electricity & heat generation, transportation, and industrial sectors together produce more than 80% of the energy-related CO₂ emissions. The solutions proposed for mitigating the effects of climate change are deployment of low carbon content fuels, as well as improving the efficiency of the current technologies for energy conversion. Utilizing hydrogen as an alternative fuel in the industrial, residential, and transportation sectors can help achieve these goals.

Hydrogen fuel from low-carbon sources is viewed as a promising long-term option for tackling CO₂ emissions from distributed sources. A major benefit of utilizing hydrogen can be seen when it is fed to a proton exchange membrane fuel cell (PEMFC), where only water is produced as a byproduct of electricity production.

The global growth rate of hydrogen demand is expected to remain constant up to 2025 at the current rate of 3.5% per year. Current annual hydrogen production capacity in U.S. refineries is approximately 15.8 million tonnes. Of this volume, 3.3 million tonnes is the captive by-product hydrogen and 2 million tonnes is combusted for heat generation. Hence, the total hydrogen capacity from the conventional captive and merchant markets is approximately 11 million tonnes per year. Around 50 million tonnes of hydrogen is produced globally each year. Approximately less than 50% of this produced volume comes from steam methane reforming (SMR), 30% from oil/naphtha reforming, 18% from coal gasification, 3.9% from water electrolysis, and the remainder from other sources such as plasma processes. Since hydrogen is primarily produced from fossil fuels, such as natural gas, CO₂ is generated and subsequently released into the atmosphere. However, sustainable hydrogen production combined with carbon capture and storage (CCS) technology could be a possible strategy for addressing CO₂ generated from the transportation sector. For instance, using an alternative technology to SMR with pressure swing adsorption (PSA) for H₂ production, such as a membrane reactor (MR), has the potential to lead to H₂ production on demand with subsequent CO₂ storage. This technology can be suitable for distributed and/or mobile hydrogen.

In recent decades, MR technology for H₂ production has advanced significantly. As a special field of interest, metallic membranes, specifically Pd and its alloys, have been heavily investigated in the hydrogen separation field because hydrogen can selectively permeate through the dense metal layer reaching 99.999+% purity. These characteristic advantages could make metallic membranes and MRs suitable for onsite hydrogen production and separation to satisfy distributed hydrogen demands for mobile and small-scale systems.

The use of Pd-based membranes is also appropriate for smaller and emerging hydrogen markets, such as the semiconductor and fuel cell industries, due to the scales and purity requirements. First, these applications are small- to medium-scale with capacities ranging from 5 to 1,000 m³/h, more than 100 times smaller than large-scale applications such as refining and ammonia production. The inherent compactness of membrane units would enable on-site production of hydrogen for applications on these scales. Alternatively, distributed small-scale users can source hydrogen from large-scale plants at a reasonable production cost. However, the high cost of distribution would make this option less attractive.

The distribution cost of hydrogen is 15 times higher than that of liquid hydrocarbon fuels on a mass basis primarily due to higher pumping costs. If CCS is considered, CO₂ distribution to sequestration sites would contribute to the cost of small-scale production units. In this case, Sjardin et al. concluded from a techno-economic study that at a hydrogen production scale below 40 MW (ca. 12,300 m³/h), MRs with CCS ($19/GJ) would become competitive with centralized H₂ production ($18/GJ).

On-site hydrogen production would also provide self-sufficient supply and circumvent delivery delays as well as issues with storage safety. Nevertheless, higher production costs due to smaller product sizes and safety concerns over local production and handling of hydrogen as an explosive gas should be taken into consideration. Moreover, fuel cell and semiconductor applications require hydrogen of very high purities. In semiconductor manufacturing, hydrogen is used as a carrier gas for trace doping elements and hydrogen of near absolute purity is needed. For PEMFCs, CO impurities in the hydrogen feed could strongly adsorb onto the catalyst, leading to catalyst deactivation and cell performance deterioration.

Therefore, despite several decades of research, most metallic membranes have remained at the pilot scale due to uncertain economic feasibility and competitiveness at a large scale. Although the scientific literature on this topic is exceptionally rich, only few scientific studies have focused on the analysis of the costs and challenges associated with industrial feasibility.